The use of carbon dioxide (CO2) spray to remove particles sized from micrometers to as small as nanometers from surfaces has emerged in cleaning technology as an acceptable replacement for Freon cleaning. Effective delivery of CO2 will remove the particulate contamination and organic films (light oils, silicone lubricants, etc.) by momentum transfer between the CO2 snow and contaminant.
CO2 snow is used to clean for example optical components. In general, the CO2 jet needs to be controlled in its combination of solid snow called “pellets” of CO2 (i.e. dry ice) and gaseous CO2. The CO2 snow properties such as size, velocity, density and flux can be controlled by the design of the nozzle employed, as well as by other characteristics of the CO2 at the CO2 source such as pressure and temperature. Carbon dioxide snow cleaning is dry, nonabrasive, chemical-free and residue-free, thus making this cleaning procedure attractive for many critical cleaning applications.
in a conventional nozzle, the nucleation, growth and compacting of CO2 snow takes place after expansion through an orifice separating inlet liquid CO2 (at approximately 800 psi) from the expanding gas phase in the downstream cavity of the nozzle (so called barrel). The CO2 plume contains snow pellets with sizes that can exceed 50 micrometers (μm) in diameter, and yet still have velocities comparable to smaller size snow pellets. These larger pellets (whether they are CO2 or a contaminant initiating from the CO2 source tank) travel with a momentum sufficiently large to damage tine structures of the workpiece being cleaned or processed, if the number and momentum of the particles additively exceeds the device damage threshold value.
CO2 molecules can coalesce or agglomerate onto a CO2 assembly. This agglomeration of the CO2 molecules occurs during passage through the orifice or directly after, where the liquid CO2 converts to the CO2 gas phase in the downstream barrel-like tube. Additionally, contaminants may build up or accumulate on the wall of the barrel along with the CO2 agglomerates, “Adders” result from contaminants on the CO2 source which accumulate from agglomeration in the nozzle and then are deposited on a surface of the workpiece or wafer being cleaned. The adders are thus transported in the CO2 stream from the nozzle onto the object or surface to be cleaned. Such adders when discharged from known nozzles can cause contamination by adhering to the very surface that the nozzle is being employed to clean and can possibly damage the surface as well. Mitigating the agglomeration at the barrel interior surface would correspondingly reduce if not eliminate the adders and problems associated therewith.
To overcome this deficiency, it is known to purify a CO2 reservoir to less than parts per billion (“ppb”) if at all possible, and to clean and degrease an interior surface of the nozzle of lubricants which were used during machining and drilling by electropolishing, extrusion techniques, etc., to construct the nozzle. Of course, some residue, such as a film layer, may remain from nozzle fabrication disposed at an inner surface of the nozzle, which residue is reduced to an extent by baking-out the nozzle.